An optical pickup focuses a light beam onto a recording surface of an optical disk through an objective lens. The objective lens is installed in an objective lens holding device. The holding device which is controlled according to a varying amount of a detected light beam reflected from the optical disk so that the focused light beam accurately traces each track on the recording face.
The objective lens is secured to a lens holder in the objective lens holding device. In this arrangement, the lens holder is driven in either of two directions: a focusing direction and a tracking direction. The focusing direction is identical to the light-axis direction of the light beam that is perpendicular to the recording surface of the optical disk, and the tracking direction is identical to the radial direction of the optical disk.
In such an objective lens holding device, it is necessary to drive the objective lens in the tracking direction and in the focusing direction without causing any deviation between the axis of the objective lens and the light axis so that each track of the recording surface is precisely traced.
For this reason, such devices have been known in the art and are characterized by a parallel link mechanism for moving the lens holder with the objective lens in the focusing direction and a pivot mechanism for moving the objective lens in the tracking direction. These mechanisms are installed so as to prevent any deviation between the axis of the objective lens and the light axis.
The parallel link mechanism includes a pair of plate-shaped links facing each other in the focusing direction. Each link is provided with hinges at both corners of the fixed edge and at both corners of the pivoting edge. A rocking vertical plate is secured to the plate-shaped links by means of the hinges secured to both corners of the pivoting edge
These parallel link mechanism and pivot mechanism are integrally molded from synthetic resin to make a mold-type support member for the purpose of improving working efficiency of assembly and achieving low cost.
Each of the focusing-use hinges in the parallel link mechanism has elasticity since it is provided with a partially thinner portion in the focusing direction. On the other hand, the pivot mechanism includes tracking-use hinges each of which has elasticity with a partially thinner portion and also has a pivotal axis that is parallel to the focusing direction. Thus, the lens holder is supported at a predetermined position with each of the focusing-use hinges being elastically deformed.
In order to drive the lens holder in such a manner, a focusing-use coil and a tracking-use coil are installed in the lens holder, and a magnetic circuit for producing magnetic fields that permit those coils to generate driving forces is installed in the optical pickup.
When the objective lens is attached to the lens holder, the adjustment of the optical pickup is performed so that the objective lens is located at an optimum position with respect to the axis of the light beam, for example, by aligning the light axis with the axis of the objective lens.
For the optical pickup thus adjusted as described above, it is desirable that the axis of the objective lens should not deviate from the light axis, that is, should not be tilted therefrom, under any changes of environmental conditions from high temperature to low temperature or from low temperature to high temperature.
The objective lens holding devices adopting the above-mentioned mold-type support member have many advantages such as high working efficiency of assembly and low cost. However, they also have a disadvantage in that the axis of the objective lens tends to deviate from the light axis due to temperature changes.
For example, in the case of conducting shelf tests under high temperatures, since the focusing-use hinges are made of thin resin, they will softened and cause the optical pickup to deteriorate in performance as is described below.
More specifically, when an optical pickup, whose objective lens has been adjusted so as to be located at an appropriate position with respect to the light axis, is left under high temperature, the focusing-use hinges will softened and cause the lens holder including those coils and objective lens to dislocate due to its weight. If this state is left untreated, the bottom surface of the lens holder will contact the inner wall of a housing.
In such case, the inner wall has been formed without taking account of the light axis although the objective lens has been installed and adjusted with respect to the light axis. Therefore, when the lens holder contacts the inner wall, it is often kept in a slanted state not parallel to the original state, that is, in a distorted state.
If the ambient temperature drops to normal temperature or low temperature after the lens holder has been kept in the distorted state, the focusing-use hinges will recover their original modulus of elasticity and the lens holder is moved to its original state. However, the lens holder will actually return to a somewhat different state from the original state (i.e., before it underwent the high temperature).
Therefore, the main problem of the conventional arrangement is that deviation and tilt might occur between the axis of the objective lens and the light axis due to temperature changes, resulting in serious adverse effects on the optical performance.
Conventionally, in order to avoid the deterioration in optical performance due to temperature changes, it has been suggested to enhance the degree of parallelism between the inner walls of those parts, such as the base plate, located in the housing and the bottom surface of the lens holder. However, achieving such precision in machining necessitates highly precise machining and adjusting processes. But such precision jeopardizes the cost reduction which otherwise would be achieved by adopting the mold-type support member.
Another possible method for solving the above problem involves heating the whose device to a high temperature before the adjusting process for attaching the objective lens to the lens holder. Then the adjusting process is conducted. In this method, it is possible to adjust the positional relationship between the objective lens and the light axis so that serious deviation is not caused in the relationship even after the lens holder has come into contact with the base plate under high-temperature states occasioned thereafter. However, such an adjusting method causes problems in that time-consuming processes are required and the additional process for heating the whole process has to be provided, resulting in a high manufacturing cost.
Furthermore, conventionally, once the lens holder has been brought into a distorted state due to temperature changes to cause the deviation of the mounting station of the objective lens, the adjustment of the mounting station of the objective lens is again conducted.